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Anywhere between 10x-20x turnover per hour is often recommended as a good point to shoot for. Some fish and corals prefer less movement than others, some more than others, so it also depends on what you have in your tank. Do you know the rating of the powerhead you are currently running?
Can't tell you. It'll be based on the size of the pump and shape of your tank. You're gonna have to play with it to find out what's best for your environment.